The official Twitter client for iOS is pretty good, and it integrates directly with the OS. But Tweetbot is still the best client around, not due to any one specific feature, but just due to how well the app is put together. It's found that perfect balance between being easy to use and being feature-filled, which is a hard sweet spot to hit. If you haven't tried it out yet—and you don't like the new interface of Twitter's official app—now's a good time to check out Tweetbot.